Q17: How do you perform Level Order Traversal (BFS) on a Binary Tree?

Use Breadth-First Search (BFS) with a Queue<TreeNode>. Offer root to queue. While queue is not empty, measure levelSize = queue.size(). Loop levelSize times to process nodes on the current level, polling nodes and offering non-null child nodes.

Level Order Traversal Solution

import java.util.*;

public class LevelOrder {
    public static List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            int levelSize = queue.size();
            List<Integer> currentLevel = new ArrayList<>();
            for (int i = 0; i < levelSize; i++) {
                TreeNode node = queue.poll();
                currentLevel.add(node.val);
                if (node.left != null) queue.offer(node.left);
                if (node.right != null) queue.offer(node.right);
            }
            result.add(currentLevel);
        }
        return result;
    }
}

💡 Uses queue snapshot sizes to partition nodes into level-by-level list arrays.

Key Takeaways

BFS traversal utilizes a Queue data structure.
Process levels in batches using snapshot level sizes (queue.size()).

Knowledge Check

Why is recording levelSize = queue.size() BEFORE the level for-loop critical?

Q17: How do you perform Level Order Traversal (BFS) on a Binary Tree?

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